Burnish resistant printing sheets

ABSTRACT

A coated printing sheet is provided that is suitable for conventional offset printing grades, exhibiting desirable surface and optical properties and providing a surface that is image receptive and resistant to coating failure. The coated printing sheet includes an image receptive coating containing a hard polymer pigment having a shear modulus of at least 5.0×10 9  dynes/cm 2  and a film forming binder. The coated printing sheets resist burnishing and exhibit desirable properties, e.g., gloss, bulk, stiffness and smoothness, with minimal or no calendering.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a coated printing sheet. Thepresent invention further relates to methods of manufacturing such acoated printing sheet.

[0002] Coated printing papers are typically required to meet manyproduct attribute and performance characteristics. The surface finish,e.g., glossy, dull or matte, and related product quality characteristicsof a coated printing sheet are generally dictated by the end use. Forexample, printed material comprising primarily text is typically printedon paper having a dull or matte finish which facilitates reading;conversely, printed material comprising mostly images, such asmagazines, is generally printed on paper having a very glossy finishwhich tends to accentuate the images.

[0003] High quality coated printing papers, regardless of surfacefinish, are required to meet certain optical properties to ensure thatthe final printed product exhibits the desired image quality. Highquality printing sheets tend to exhibit high brightness whichaccentuates the color reproduction of the printed images. Because mostprinting sheets are printed on both sides, the opacity of the printingsheet should be sufficient to reduce show-through of printed text orimages from one side of the sheet to the other side.

[0004] Other product attributes may affect the performancecharacteristics of the printing sheet. Smoothness of the sheet tends toenhance the reproduction of images and the clarity of text. Coatedprinting sheets should exhibit an adequate level of porosity to absorbink solvents, and in the case of offset lithography, the fountainsolution. Coated printing sheets should exhibit a sufficient level ofstrength and stiffness to withstand printing and any subsequentfinishing processes, such as trimming and binding. Printers typicallydemand printing sheets with relatively high bulk and stiffniess tomaintain printing press runnability.

[0005] Manufacturers of high quality printing sheets generally employsome form of calendering after coating to achieve an appropriate levelof paper gloss and smoothness. Increased levels of calendering tend toproduce higher gloss and greater smoothness. However, calendering alsotends to reduce opacity, stiffness and bulk. Thus, efforts to improvegloss and smoothness through calendering can negatively affect theproperties printers desire for runnability.

[0006] In addition, high levels of calendering can cause undesirablemottling problems in the paper product and in the final printed image.There are several types of mottling problems, i.e., microgloss andbacktrap mottle, which are related to nonuniformities in the paper web.Calendering magnifies these nonuniformities, thereby negativelyaffecting paper surface quality and final printed image quality.Consequently, paper manufacturers tend to select calendering conditionsthat optimize certain properties and minimize aesthetically undesirableeffects; in doing so other desirable properties are often sacrificed.

[0007] Typically, printing papers designed to exhibit low levels ofgloss, such as dull or matte grades of paper, are not calendered or arecalendered very lightly. Uncalendered printing sheets are particularlysusceptible to burnishing, i.e., localized areas of increased gloss orreflectivity on the surface of the sheet typically caused by mechanicalrubbing. Uncalendered printing sheets also tend to exhibit greaterlevels of porosity, which may exacerbate a variety of printing problemsif the ink solvent or carrier drains too quickly into the coated surfacelayer. An overcoat varnish may be applied after printing to protect thepaper surface and minimize burnishing but such steps typically addmanufacturing complexity and undesirable cost to the final printedproduct.

[0008] To avoid the deleterious effect of calendering, the use ofall-latex coatings has been proposed for glossy sheets. Because anall-latex coating tends to form a continuous film on the surface of thesheet, surface gloss tends to be very high. The porosity of such latexcoated sheets, however, tends be very low which results in increased inksetting, i.e., the amount of time necessary for the ink on the surfaceof the coating to dry, or set, sufficiently to allow physical handling,which tends to reduce the quality of the final printed image and tocreate production inefficiencies. Such glossy sheets also tend toexhibit burnishing.

[0009] There remains a need for an offset printing sheet that exhibitsthe product attributes desired by printers and publishers without theaesthetically undesirable effects discussed above. Specifically thereremains a need for an offset printing sheet that exhibits the productattributes typically achieved through calendering without the negativeeffects of calendering. In addition, there remains a need for anuncalendered printing sheet that does not exhibit the undesirablecharacteristics of uncalendered sheets, such as burnishing and highporosity.

SUMMARY OF THE INVENTION

[0010] The inventor has discovered that including a hard polymer pigmenthaving a shear modulus of at least 5.0×10⁹ dynes/cm² and a film formingbinder in an image receptive coating provides a printing sheet thatexhibits the surface and optical properties expected for conventionaloffset printing grades, and provides a surface that is image receptiveand resistant to coating failure or picking, i.e., localizeddelamination of the coating layer from the underlying substrate, duringthe manufacturing process and/or during printing. The term “shearmodulus,” as used herein, means the elastic, or storage, modulus ofpolymeric material as determined by dynamic mechanical analysis,measured at approximately room temperature, e.g., 21° C. The coatedprinting sheets resist burnishing, i.e., localized areas of increasedgloss or reflectivity on the surface of the sheet typically caused bymechanical rubbing. Without intending to be bound by any particulartheory, the burnish resistance appears to be related to the resistanceto deformation exhibited by the hard polymer pigment particles.Generally, the coated printing sheets exhibit desirable properties,e.g., gloss, bulk, stiffness and smoothness, with minimal or nocalendering.

[0011] Preferably the image receptive coating of the printing sheetsprovides sufficient ink drainage or ink setting, i.e., a proportion ofthe ink solvent carrier drains into the image receptive coating suchthat the ink on the surface of the coating dries, or sets, sufficientlyto allow physical handling of the printed sheet within a relativelyshort period of time, e.g., 30 to 45 minutes. Ink setting isdistinguished from true ink drying which is caused by the completeremoval of the solvent and the resulting oxidation of the ink. Thecoating of the coated printing sheets also exhibits sufficient inktransfer, i.e., absorption of the ink-fountain solution mixture by theimage receptive surface is such that a uniform film of ink istransferred from the printing blanket to the sheet during offsetprinting, and sufficient ink holdout, i.e., the printing ink remains onthe surface of the coating. Ink setting, ink transfer and ink holdoutaffect final product attributes such as the ink gloss and sharpness ofthe printed image.

[0012] In one aspect, the invention provides a printing sheet includinga substrate and, on at least one surface of the substrate, an imagereceptive coating including a film forming binder and a hard polymerpigment having a shear modulus of at least 5.0×10⁹ dynes/cm².

[0013] Preferred embodiments may include one or more of the followingfeatures. The hard polymer pigment has a shear modulus of at least10.0×10⁹ dynes/cm². The hard polymer pigment is essentially non-filmforming and remains in the form of discrete roughly spherical solidparticles. The hard polymer pigment has a glass transition temperature(T_(g)) of at least 80° C., preferably at least 105° C. The hard polymerpigment is selected from the group consisting of poly(methylmethacrylate), poly(2-chloroethyl methacrylate), poly(isopropylmethacrylate), poly(phenyl methacrylate), polyacrylonitrile,polymethacrylonitrile, polycarbonates, polyetheretherketones,polyimides, acetals, polyphenylene sulfides, phenolic resins, melamineresins, urea resins, epoxy resins, and alloys, blends, mixtures andderivatives thereof. The hard polymer pigment has a homogenouscomposition comprising poly(methyl methacrylate) particles. The hardpolymer pigment particles have a particle size of less than about 2,000angstroms (Å), preferably less than about 1,500 Å, more preferably aparticle size ranging from about 600 to 1,200 Å. The image receptivecoating includes at least 30 parts by weight of the hard polymerpigment, preferably at least 50 parts, more preferably at least 80parts, based on 100 parts by weight of total pigment. The term “parts,”as used herein, means parts on a dry solids basis, and, as is well knownin the art, parts are based on 100 parts of pigment. The film formingbinder is selected from the group consisting of latex, starch,polyacrylate salt, polyvinyl alcohol, soy, casein, carboxymethylcellulose, hydroxymethyl cellulose and mixtures thereof. Preferably thefilm forming binder is a latex selected from the group consisting ofstyrene-butadiene, styrene-butadiene-acrylonitrile, styrene-acrylic,styrene-butadiene-acrylic and mixtures thereof. The image receptivecoating includes 5 to 75 parts by weight of film forming binder, basedon 100 parts by weight of total pigment. The image receptive coatingfurther includes a pigment selected from the group consisting ofstructured polymer pigment, kaolin, calcined clay, structured clay,ground calcium carbonate, precipitated calcium carbonate, titaniumdioxide, aluminum trihydrate, satin white, hollow sphere plasticpigment, solid plastic pigment, silica, zinc oxide, barium sulfate andmixtures thereof. The image receptive coating further includes astructured polymer pigment consisting of a soft domain having a glasstransition temperature of less than about 50° C. and a hard domainhaving a glass transition temperature of greater than about 55° C. Theimage receptive coating has a total dried coat weight per side of about1 to 4 g/m².

[0014] The substrate, prior to application of the image receptivecoating, has a smoothness of less than about 3.5 μm, preferably lessthan about 2.0 μm, more preferably less than about 1.5 μm. The printingsheet further includes at least one precoat layer on the first surfaceof the substrate underlying the image receptive coating layer. Theprecoat layer includes a binder and a pigment selected from the groupconsisting of kaolin, calcined clay, structured clay, ground calciumcarbonate, precipitated calcium carbonate, titanium dioxide, aluminumtrihydrate, satin white, hollow sphere plastic pigment, solid plasticpigment, silica, zinc oxide, barium sulfate and mixtures thereof. Thepigment component of the precoat has a monodisperse particle sizedistribution. Preferably the monodisperse pigment is selected from thegroup consisting of precipitated calcium carbonate, hollow sphereplastic pigment and mixtures thereof. The precoat layer has a totaldried coat weight per side of about 5 to 15 g/m².

[0015] In another aspect, the invention features a printing sheetincluding a substrate and, on at least one surface of the substrate, animage receptive coating including a film forming binder and a hardpolymer pigment, wherein the hard polymer pigment is essentiallynon-film forming and remains in the form of discrete roughly sphericalsolid particles.

[0016] In another aspect, the invention provides a method ofmanufacturing a printing sheet including:

[0017] a) applying an image receptive coating, including a hard polymerpigment having a shear modulus of at least 5.0×10⁹ dynes/cm² and a filmforming binder, to at least a first surface of a substrate; and

[0018] b) drying the image receptive coating layer.

[0019] Preferred methods may include one or more of the followingfeatures. The hard polymer pigment has a shear modulus of at least10.0×10⁹ dynes/cm². A pressing step is performed on the substrate beforethe application of the image receptive coating at a moisture levelranging from 20 to 60% and at a temperature of at least 100° C. Aprecoating step and a precoat drying step are performed beforeapplication of the image receptive coating. A calendering step isperformed before the application of the image receptive coating. Acalendering step is performed after the image receptive coating dryingstep, preferably at a nip pressure ranging from 40 to 90 kN/m and apaper surface temperature of at least 5° C. lower than the glasstransition temperature of the hard polymer pigment. A brushing step isperformed after the image receptive coating drying step.

[0020] Other features and advantages of the invention will be apparentfrom the following detailed description, the drawing, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021]FIG. 1 is an SEM photomicrograph of a top view of the imagereceptive surface of an embodiment of the invention.

[0022]FIG. 2 is a graph of shear modulus, G′, as a function oftemperature.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0023] Preferably the image receptive coating of the printing sheets ofthe invention includes a hard polymer pigment having a shear modulus ofat least 5.0×10⁹ dynes/cm² and a film forming binder.

[0024] Suitable hard polymer pigments exhibit a shear modulus of atleast 5.0×10⁹ dynes/cm², preferably at least 10.0×10⁹ dynes/cm². Theshear modulus exhibited by suitable hard polymer pigments generallyallows the particles to resist deformation during the papermanufacturing process. The resistance to deformation appears to berelated to the burnish resistance exhibited by the printing sheets ofthe invention.

[0025] The hard polymer pigment particles in the image receptive coatinglayer are essentially non-film forming and typically remain in the formof discrete roughly spherical solid particles in the image receptivecoating layer throughout the paper manufacturing process and in thefinal printing sheet product. The term “essentially non-film forming,”as used herein, means that the hard polymer particles will not form acontinuous film under the temperature conditions used to dry the imagereceptive layer. A suitable hard polymer pigment typically exhibits aglass transition temperature of at least 80° C., preferably at least105° C., exceeding the highest temperature to which the coating will besubjected during the manufacturing process. The non-film forming natureof the hard polymer pigment is in part a consequence of the relativelyhigh glass transition temperature.

[0026] In addition, the hard polymer pigment particles typically remainin the form of discrete roughly spherical solid particles because theytend to be resistant to deformation under the pressures encounteredduring the manufacture of the printing sheet. FIG. 1 is an SEMphotomicrograph of the image receptive surface of a printing sheet ofthe invention, containing hard polymer pigment as the sole pigment. Thehard polymer pigment particles are clearly visible as discrete roughlyspherical particles in FIG. 1.

[0027] It is advantageous that the hard polymer pigment be non-filmforming because it functions as a pigment in the image receptivecoating. This non-film forming characteristic allows the hard polymerpigment to provide smoothness to the final dried coating layer withoutthe need for calendering as well as sufficient porosity for satisfactoryink setting. Because the hard polymer pigment particles typically remainin the form of discrete roughly spherical particles, the resultingcoating layer is discontinuous, providing the porosity necessary forsatisfactory ink setting. The discontinuous nature of the imagereceptive coating layer is evident in FIG. 1.

[0028] Suitable hard polymer pigments may have a homogeneous orheterogeneous composition including, for example, hard acrylic resins(e.g., poly(methyl methacrylate) (PMMA), poly(2-chloroethylmethacrylate), poly(isopropyl methacrylate), poly(phenyl methacrylate),polyacrylonitrile, polymethacrylonitrile, etc.), polycarbonates,polyetheretherketones (PEEK), polyimides, acetals, polyphenylenesulfides, and alloys, blends and derivatives thereof, and certain hardpolymer resins such as phenolic resins, melamine resins, urea resins,and epoxy resins. A hard polymer pigment may take the form of aheterogeneous structured polymer pigment as described below provided thestructured polymer pigment exhibits a shear modulus of at least 5.0×10⁹dynes/cm².

[0029] Preferably the hard polymer pigment includes hard acrylic resins,more preferably PMMA. Suitable PMMA pigments are commercially availablefrom Specialty Polymers, Inc. located in Oregon, e.g., H30S-PC.

[0030] Hard homogeneous polymer pigments are typically prepared byemulsion polymerization. Hard heterogeneous polymer pigments aretypically prepared in a sequential or staged emulsion process in which afirst polymer is initially prepared in a first-stage emulsionpolymerization. Thereafter, a second polymer is formed in a second-stageemulsion polymerization in the presence of the first polymer resultingfrom the first-stage polymerization. The methods of manufacture ofhomogeneous and heterogeneous polymers are known in the art, e.g., asdisclosed in U.S. Pat. Nos. 4,478,974, 4,134,872, 5,308,890 and4,613,633, the disclosures of which are incorporated herein byreference.

[0031] The particle size of preferred hard polymer pigments is generallywithin the size range typically used for pigments in image receptivecoatings. Preferably, the particle size of hard polymer pigments rangesfrom less than about 2,000 angstroms (Å), more preferably less thanabout 1,500 Å, most preferably about 600 to 1,200 Å. Hard polymerpigments having smaller particle size tend to improve gloss andsmoothness, thereby reducing or eliminating the need for calendering.Hard polymer pigments having particle sizes greater than about 1,500 Åtend to result in reduced paper gloss but generally maintain desiredlevels of ink gloss and smoothness. Thus, hard polymers pigments havingparticle sizes greater than about 1,500 Å may be used when reduced glosslevels are desired, e.g., dull or matte grades of paper.

[0032] The hard polymer pigment may be used as the sole pigment in theimage receptive layer or it may be used with other organic or inorganicpigments. The image receptive coating generally includes at least 30parts by weight of the hard polymer pigment, based on 100 parts byweight of total pigment. Preferably, the image receptive coatingincludes at least 50 parts by weight, more preferably at least 80 partsby weight, based on 100 parts by weight of total pigment.

[0033] The film forming binder component of the image receptive coatingmay include latex, starch, polyacrylate salt, polyvinyl alcohol, protein(e.g., soy, casein), carboxymethyl cellulose, hydroxymethyl celluloseand mixtures thereof. Suitable starches include pearl, ethylated,oxidized or enzyme treated starch, all of which may be derived frompotato, corn, rice or tapioca starches. Preferably the coating binder isa latex. Typical monomers used in the production of latex polymers forpaper coatings include styrene, butadiene, acrylonitrile, butylacrylate, methyl methacrylate, vinyl acrylate, isoprene and combinationsthereof. Preferred latexes include styrene-butadiene,styrene-butadiene-acrylonitrile, styrene-acrylic,styrene-butadiene-acrylic and mixtures thereof.

[0034] Preferably the amount of film forming binder in the coating isabout 5 to 75 parts by weight based on 100 parts of pigment. The amountof binder in the coating should provide adequate coating strength toresist picking. The mean particle size of the latex particles typicallyused as film forming binders for the manufacture of coated printingsheets is generally about 800 to 2,400 angstroms. Coatings with smallerlatex particles typically exhibit improved coating strength becausesmaller particles provide a greater surface area per unit weight withwhich to bind the other coating components. Examples of suitable latexesinclude: CP 620NA and CP 615NA, manufactured by The Dow ChemicalCompany; GenFlo 557 and GenFlo 576, manufactured by Omnova SolutionsInc.; and Acronal S 504 and Acronal S 728, manufactured by BASFCorporation.

[0035] The image receptive coating may further include structuredpolymer pigment particles in addition to the hard polymer pigment. Asdiscussed above, the hard polymer pigment may also take the form of aheterogeneous structured polymer pigment provided the structured polymerpigment exhibits a shear modulus of at least 5.0×10⁹ dynes/cm². However,if the structured polymer pigment is used in addition to the hardpolymer pigment, it may have any desired level of shear modulus.

[0036] The structured polymer pigment particles are broadlycharacterized as generally heterogeneous, having a portion thereofcomprising an essentially film forming “soft” polymer domain and havinga portion thereof comprising an essentially non-film forming “hard”polymer domain. As used herein, the term “domain” refers to discreteregions within the heterogeneous polymer which are either the softpolymer or the hard polymer. As used herein, the term “essentially filmforming” denotes the property of the soft polymer to form a continuousfilm under the temperature conditions used to dry coating compositionscoated on a substrate.

[0037] When the structured polymer pigment is used as an additive to theimage receptive coating, it is advantageous that the structured polymerpigment be non-film forming because it functions as a pigment in theimage receptive coating. This non-film forming characteristic allows thestructured polymer pigment to provide smoothness to the final driedcoating layer without the need for calendering as well as sufficientporosity for satisfactory ink setting. Without intending to be bound bya particular theory, the ability to provide smoothness withoutcalendering and porosity appears to be related to limited coalescence ofthe structured polymer pigment when subjected to typical dryingconditions after coating. The term “limited coalescence” as used herein,means that, during the drying process, the non-film forming hard domainsof the structured polymer particles maintain their structure while thefilm forming soft domains of the particles coalesce. Because thestructured polymer pigment particles typically remain in the form ofdiscrete roughly spherical particles, the resulting coating layer isdiscontinuous, providing the porosity necessary for satisfactory inksetting. Greater porosity also tends to improve the ability of moisturewithin the printing sheet to escape during web printing. When moisturecannot escape rapidly, blistering, i.e., disruptions in the printedimage caused by the delamination of the coating layer from theunderlying substrate, may result.

[0038] Typical synthetic polymer latexes completely coalesce duringdrying or calendering, forming a continuous film. Complete coalescence,or complete film formation, tends to cause a decrease in porosity whichin turn increases ink setting time.

[0039] The extent of coalescence of a coating containing a structuredpolymer pigment may be measured indirectly by measuring porosity, abroad term used to indicate both air permeance, i.e., amount of air thatflows through a paper specimen using the Sheffield method, and airresistance, i.e., the amount of time it takes a specific volume of airto pass through a given surface area using the Gurley method. Increasedlevels of coalescence tend to cause decreases in Sheffield porosity ofthe coating. Higher Sheffield values indicate higher porosity whilehigher Gurley values indicate lower porosity. Photomicrographs mayprovide a qualitative indication of porosity and, consequently, anindication of the extent of coalescence. An indication of the limitedcoalescence may also be derived from ink absorption tests, e.g., K&N inkabsorption test. Increased coalescence tends to result in reduced K&Nink absorption.

[0040] The distribution within a heterogeneous structured polymerpigment particle of the soft polymer domain and the hard polymer domaincan vary. For example, the heterogeneous particle may have only twodistinct regions, e.g., mutually exclusive hemispherical soft and hardregions. On the other hand, the heterogeneous particle may have multipleregions of one or both components. For example, a generally sphericalcontinuous region of one polymer may have several discrete regions ofthe other polymer dispersed in, or residing on the surface of, thecontinuous region. Alternatively, the heterogeneous particle may have anessentially continuous web-like region of one polymer that has itsinterstices filled with the other polymer. The structured polymerpigment particle may also exhibit a core/shell morphology, i.e., thecore polymer is encapsulated within the shell polymer, the particleshaving one core or a multiplicity of cores.

[0041] Preferably, the distribution within the heterogeneous structuredpolymer pigment is such that the hard polymer domain is in the form of acontinuous matrix having discrete regions of the soft polymer domaindispersed within such hard matrix and/or distributed on the surfacethereof.

[0042] The amount of the hard domain regions in 100 parts by weight ofthe heterogeneous structured polymer pigment particles is from about 55to 90 parts by weight, preferably from about 60 to 70 parts by weight.The amount of the soft domain regions in 100 parts by weight of theheterogeneous structured polymer pigment particles is from about 10 to45 parts by weight, preferably from about 30 to 40 parts by weight. Ifthe amount of the hard domain regions in the structured polymer pigmentis greater than about 90 parts by weight, the structured polymer pigmentmay not exhibit adequate film forming characteristics, and the imagereceptive coating layer tends to be too porous. If the amount of thesoft domain regions in the structured polymer pigment is greater thanabout 45 parts by weight, the structured polymer pigment tends toexhibit the film forming characteristics of conventional latexes and theimage receptive coating layer tends to exhibit complete coalescence.

[0043] Each domain of the structured polymer pigment exhibits a distinctglass transition temperature (T_(g)), i.e., second order thermodynamictransition temperature of a semicrystalline polymer wherein the polymertransitions from a glass state to a rubbery state. Consequently,structured polymer pigments suitable for use in the invention exhibitmultiple glass transition temperatures. Preferably, the structuredpolymer pigment exhibits at least two distinct glass transitiontemperatures, corresponding to the hard and soft domains of thestructured polymer pigment.

[0044] The T_(g) of the soft polymer domain may be higher or lower thanthe highest temperature to which the coating will be subjected duringthe manufacturing process. Preferably at least one T_(g) of thestructured polymer particle, exhibited by the soft polymer domain,should be lower than the highest temperature to which the coating willbe subjected during the manufacturing process, preferably at least 10°C. lower, to provide additional coalescence to the image receptivecoating beyond that which is provided by the binder component alone. Forexample, if the surface temperature of the paper web in a particularprocess reaches 80° C., the T_(g) of the hard domain should exceed 80°C., preferably exceed 85° C., and the T_(g) of the soft domain should belower than 80° C., preferably below 70° C. In this manner, the softdomain of the structured polymer pigment ensures that some coalescenceoccurs during drying, while the hard domain of the structured polymerpigment ensures that the discrete particulate form is maintained. Thus,the coalescence of the structured polymer pigment is-limited and theresulting image receptive coating layer is discontinuous.

[0045] The structured polymer pigment particles may include one or moredistinct hard polymer domains and one or more distinct soft polymerdomains. Each hard and soft domain typically exhibits a T_(g). Thus, thestructured polymer pigment may exhibit more than two T_(g)'s if itincludes more than one hard and/or soft polymer domain.

[0046] The structured polymer pigments used in the invention areadvantageously prepared in a sequential or staged emulsion process inwhich a polymer of either of the aforementioned soft or hard domains isinitially prepared in a first-stage emulsion polymerization. Thereafter,the remaining hard or soft domain is formed in a second-stage emulsionpolymerization in the presence of the hard or soft polymer resultingfrom the first-stage polymerization. The methods of manufacture ofstructured polymers are known in the art, e.g., as disclosed in U.S.Pat. Nos. 4,478,974, 4,134,872, 5,308,890 and 4,613,633, the disclosuresof which are incorporated herein by reference.

[0047] Monomers used for the production of the soft domain of thestructured polymer pigment include, for example, aliphatic conjugateddiene monomers such as 1,3-butadiene, 2-methyl-1,3-butadiene,2,3-dimethyl-1,3-butadiene, 1,3-pentadiene; 2-neopentyl-1,3-butadieneand other hydrocarbon analogs of 1,3-butadiene, and, in addition, thesubstituted 1,3-butadienes, such as 2-chloro-1,3-butadiene,2-cyano-1,3-butadiene; substituted conjugated pentadienes; conjugatedhexadienes, and mixtures thereof; isoprene; and acrylic acid esters suchas methyl acrylate, ethyl acrylate, butyl acrylate and 2-ethylhexylacrylate. These monomers may be used singly or in combination.

[0048] Monomers used for the production of the hard domain of thestructured polymer pigment include, for example, monovinylidene aromaticmonomers such as styrene, α-methylstyrene, 2-methylstyrene,3-methylstyrene, 4-methylstyrene, 2,4-diisopropylstyrene,2,4-dimethylstyrene, 4-t-butylstyrene, 5-t-butyl-2-methylstyrene,monochlorostyrene, dichlorostyrene, monofluorostyrene andhydroxymethylstyrene; methacrylic or chloroacrylic acid esters such asmethyl methacrylate, ethyl methacrylate, isopropyl methacrylate, phenylmethacrylate, cyclohexyl methacrylate, 2-chloroethyl methacrylate,methyl chloroacrylate, ethyl chloroacrylate and butyl chloroacrylate;ethylenic nitrile compounds such as acrylonitrile and methacrylonitrile;vinyl chloride; and unsaturated carboxylic acids, or esters or sodium orammonium salts thereof, such as acrylic acid, methacrylic acid, crotonicacid, cinnamic acid, itaconic acid, fumaric acid, maleic acid,butenetricarboxylic acid, and monobutyl itaconate. These monomers may beused singly or in combination. Monomers yielding film forming polymers,e.g., aliphatic conjugated diene monomers such as 1,3-butadiene,2-methyl-1,3-butadiene and 2-chloro-1,3-butadiene, and acrylic acidesters such as methyl acrylate, ethyl acrylate, butyl acrylate and2-ethylhexyl acrylate, can also be used if the copolymerization thereofwith the aforementioned monomers gives copolymers which do not form afilm at the highest temperature to which the image receptive coatingwill be subjected during the manufacturing process.

[0049] The selection of appropriate polymers for the domains of thestructured polymer pigment depends on the final printing propertiesdesired. For example, to slow down ink setting on the image receptivecoating, which tends to be caused by rapid drainage of the ink solventinto the coating, one or more of the polymeric constituents of thestructured polymer may incorporate a monomer, e.g., acrylic acid, whichinteracts to a lesser degree with the ink solvents.

[0050] As discussed above, when the hard polymer pigment is aheterogeneous structured polymer pigment, the composition of the hardand soft domains of the structured polymer pigment are selected suchthat the structured polymer pigment exhibits a shear modulus of at least5.0×10⁹ dynes/cm².

[0051] The soft domain of the structured polymer pigment preferablyexhibits a low T_(g) of less than about 50° C., more preferably about−10 to +50° C., most preferably 5 to 35° C. The hard domain of thestructured polymer pigment typically exhibits a high T_(g) of greaterthan about 55° C., more preferably greater than about 80° C.

[0052] The particle size of suitable structured polymer pigments isgenerally limited to the size range typically used for pigments in imagereceptive coatings. Preferably, the particle size of structured polymerpigments ranges from 500 to 5,000 angstroms (Å), more preferably 800 to2,000 Å, most preferably 800 to 1,400 Å. Structured polymer pigmentshaving smaller particle size tend to improve gloss and smoothness,thereby reducing or eliminating the need for calendering. Structuredpolymer pigments having particle sizes greater than about 2000 to 5000 Åtend to result in improved porosity but may require calendering toachieve desired levels of gloss and smoothness. Due to the methods ofmanufacture, suitable structured polymer pigments tend to exhibitmonodisperse size distributions, i.e., very narrow size distributionswith little variation from the targeted particle size. Blends ofstructured polymer pigments having different particle sizes may be usedin the image receptive coating.

[0053] The image receptive coating typically exhibits multiple glasstransition temperatures because each polymeric component provides aglass transition temperature. An image receptive coating including ahard polymer pigment and a film forming binder typically exhibits twoglass transition temperatures when the hard polymer pigment ishomogeneous, and three or more glass transition temperatures when thehard polymer pigment is heterogeneous. The further addition of astructured polymer pigment to the image receptive coating tends toresult in at least two more glass transition temperatures. When astructured polymer pigment is also used in the image receptive coating,the T_(g) of the binder may be higher or lower than the low T_(g) softdomain of the structured polymer pigment.

[0054] A typical temperature for drying the image receptive coatinggenerally falls between the glass transition temperatures of the hardpolymer pigment and the film forming binder. Preferably the T_(g) of thefilm forming binder falls within the range of −10 to +35° C., morepreferably 5 to 25° C.

[0055] The image receptive coating may further include conventionalinorganic and organic pigments in addition to the hard polymer pigment.Suitable pigments include kaolin, calcined clay, structured clay, groundcalcium carbonate, precipitated calcium carbonate, titanium dioxide,aluminum trihydrate, satin white, hollow sphere plastic pigment, solidplastic pigment, silica, zinc oxide, barium sulfate and mixturesthereof. Preferably the pigments include kaolin, ground calciumcarbonate, precipitated calcium carbonate and mixtures thereof. Theaverage particle size, e.g., 0.4 to 2.0 micrometers, and sizedistribution of these pigments are typical for pigments used as coatingpigments. Practitioners skilled in the art are aware of how to selectthe appropriate coating pigments to achieve the desired final productattributes.

[0056] The image receptive coating may include less than about 70 partsby weight of additional pigment, i.e., structured polymer pigments andconventional inorganic and organic pigments combined, based on 100 partsby weight of total pigment. Preferably, the image receptive coatingincludes less than about 50 parts by weight of additional pigment, morepreferably less than about 20 parts by weight, based on 100 parts byweight of total pigment.

[0057] The image receptive coating may further include optical-relatedcoating additives, such as colorants, tinting dyes, fluorescentbrighteners, blooming agents and mixtures thereof. Practitioners skilledin the art are aware of how to select the appropriate optical package toachieve the desired final product attributes, such as shade andbrightness.

[0058] The image receptive coating may further include coatingadditives, such as dispersants, thickeners, defoamers, water retentionagents, preservatives, crosslinkers, lubricants and pH control agents.Practitioners skilled in the art are aware of how to select theappropriate coating additives to meet manufacturing and productionobjectives, e.g., to control foam, rheology, and dusting, and to achievethe desired final product attributes.

[0059] The substrate is preferably a paper substrate of a weight andtype suitable for offset printing. The basis weight of suitablesubstrates before application of a coating layer typically ranges fromabout 35 to 325 g/m², preferably about 65 to 220 g/m². Preferably theash content of the substrate, i.e., the amount of inorganic materialincorporated within the substrate, including virgin pigment material andpigment material derived from a recycled fiber component of thesubstrate, is about 10 to 20% more preferably about 12 to 15%. If theash content of the substrate is too high, the stiffness of the substratemay decrease significantly. If the ash content is too low, the opticalproperties, e.g., opacity and brightness, of the sheet may be adverselyaffected, and the cost of production may increase.

[0060] Preferably, the substrate prior to coating with the imagereceptive layer exhibits a smoothness of less than about 3.5 μm, morepreferably the substrate has a smoothness of less than about 2.0 μm,most preferably less than about 1.5 μm, (as measured by the Parker PrintSurf instrument at the 10 kg Soft setting). A lower value indicates asmoother surface. A substrate with a smooth surface is desirable becausethe image receptive coating tends to have a relatively low coat weightand coating thickness. If a relatively rough substrate is used, theimage receptive coating may not cover the substrate surface sufficientlyand exhibit the level of smoothness desired for the printing sheet.Moreover, a subsequent calendering step may be avoided if the substrateis sufficiently smooth before the coating is applied.

[0061] The desired level of smoothness may be achieved through theapplication of one or more intermediate precoat layers underlying theimage receptive coating layer, or by smoothing the surface of thesubstrate itself. As used herein, a precoat layer is defined as anycoating layer applied between the substrate and the image receptivelayer, including but not limited to a size press layer and a base coatlayer. To smooth the surface of the substrate, the substrate may bepressed while still quite moist to improve its smoothness. To minimizeloss of bulk, pressing of the substrate should be performed at amoisture level of about 20 to 60% and at a temperature aboveapproximately 100° C. The smoothness of the substrate may also beimproved by drying the wet web against a hot, smooth surface.

[0062] Calendering of the substrate or the precoated substrate may alsobe used to achieve the desired levels of smoothness. Preferably the nippressures range from about 40 to 175 kN/m, the operating rolltemperature ranges from about 80 to 200° C., and the incoming Webmoisture is about 3 to 10%. While the smoothness of the substrate orprecoated substrate typically improves with increased calendering, otherdesirable properties, such as bulk, porosity, opacity and brightness,may be deleteriously affected. Practitioners skilled in the art areaware of how to select the appropriate calendering temperatures andpressures to achieve the desired substrate properties.

[0063] As described above, the substrate may include one or more precoatlayers to improve smoothness. A precoat layer may also enhance thesurface strength of the coating layer, e.g., to resist picking, increasecoating holdout (i.e., the ability of the coating to remain on thesurface of the substrate rather than striking into the substrate), andimprove optical properties of the final printing sheet, such as gloss,opacity and brightness. Because the image receptive coating imparts thedesired level of gloss, smoothness, and acceptable porosity, the precoatlayer may be used to provide other desirable properties, such asbrightness, opacity, and, to some extent, bulk. If multiple precoatlayers are used, the composition of the precoat layers may be differentto achieve different desirable properties. For example, a first precoatcoating composition may be designed to provide bulk (e.g., the coatingcontains precipitated calcium carbonate having a monodispersedistribution as the primary pigment), while a second precoat coatingcomposition, overlying the first precoat, may be designed to providesmoothness and brightness (e.g., the coating contains primarily finekaolin as the primary pigment).

[0064] The precoat composition may include components not typicallyused, or used to a limited extent, in the image receptive coating.Preferably, the precoat composition includes a pigment exhibiting amonodisperse distribution, i.e., a relatively narrow particle sizedistribution, such as precipitated calcium carbonate or hollow sphereplastic pigment. A preferred monodisperse distribution typically has asteepness factor of less than or equal to about 1.75. Steepness factor,as used herein, is defined as the ratio of the average diameter of 75%by weight of the pigment particles to the average diameter of 25% byweight of the pigment particles (D75/D25). The monodisperse pigment maybe the sole pigment in the precoat composition. A narrow particle sizedistribution in the coating tends to improve fiber coverage and toenhance optical properties. If the particle size distribution is toonarrow, the application of the coating to the substrate may benegatively affected, e.g., poor coat weight control and blade scratchesdue to poor water retention of the coating layer. If the particle sizedistribution is too broad, the particles exhibit more efficient packingwithin the coating layer which may lead to a more dense, less porouscoating resulting in a deterioration of fiber coverage. A monodispersepigment gives the precoat a very bulky structure, i.e., more voidsbetween the pigment particles, leading to higher brightness, opacity andbulk. Additionally, because the precoat is not constrained by glossrequirements, the particle size of the pigment may be optimized forlight scattering, e.g., opacity, rather than for light reflectance,e.g., gloss.

[0065] Suitable pigments for the precoat include kaolin, calcined clay,structured clay, ground calcium carbonate, precipitated calciumcarbonate, titanium dioxide, aluminum trihydrate, satin white, hollowsphere plastic pigment, solid plastic pigment, silica, zinc oxide,barium sulfate and mixtures thereof. Preferably the precoat includes apigment selected from the group consisting of precipitated calciumcarbonate, hollow sphere plastic pigment and mixtures thereof. Morepreferably, the precoat includes precipitated calcium carbonate as thesole pigment.

[0066] Precipitated calcium carbonates are commercially available in abroad range of surface areas, average particle sizes and particle sizedistributions. Typically the equivalent spherical diameter (ESD) of theprecipitated calcium carbonate particles is less than about 3 μm.Preferably about 80 to 95% by weight of the calcium carbonate particleshave an ESD of less than about 1 μm and the average ESD is about 0.4 to0.9 μm.

[0067] Precipitated calcium carbonates are commercially available in anarray of particle shapes. Preferably the precipitated calcium carbonateswill exhibit a rhombohedral shape. Suitable precipitated calciumcarbonates are manufactured by J. M. Huber Corporation, SpecialtyMinerals, Inc. and Imerys Pigments, Inc.

[0068] Suitable plastic pigments are available as hollow or solidspheres in a range of particle sizes and, in the case of hollow spherepigments, void volumes. Typically, the average particle size of solidplastic pigments ranges from 0.13 to 0.50 μm. Suitable solid sphereplastic pigments are commercially available from The Dow ChemicalCompany, e.g., 722HS, 788A and 756A, and from Omnova Solutions, Inc.,e.g., Lytron 2203. For hollow sphere plastic pigments, the averageparticle size typically ranges from about 0.5 to 1.0 μm with a shellthickness of about 0.06 to 0.09 μm. The hollow core diameter typicallyranges from about 0.38 to 0.82 μm, resulting in void volumes of about43% to 55%. Preferred hollow sphere plastic pigments have an averageparticle size of about 0.5 to 1.0 μm and a void volume of about 50% to55%. Suitable hollow sphere plastic pigments are commercially availablefrom Rohm and Haas Company, e.g., Ropaque HP-1055 and Ropaque HP-543P,and from The Dow Chemical Company, e.g., HS2000NA and HS3000NA.

[0069] The precoat may further include binders, such as latex, starch,polyacrylate salt, polyvinyl alcohol, protein (e.g., soy, casein),carboxymethyl cellulose, and hydroxymethyl cellulose, and coatingadditives, such as colorants, tinting dyes, fluorescent brighteners,blooming agents, dispersants, thickeners, defoamers, water retentionagents, preservatives, crosslinkers, lubricants and pH control agents.Practitioners skilled in the art are aware of how to select theappropriate binder and coating additives to meet manufacturing andproduction objectives and to achieve the desired final productattributes.

[0070] The image receptive coating is applied using typical papercoating equipment methods. Examples of suitable coating techniquesinclude an applicator means, e.g., applicator roll, fountain, jet, shortdwell, slotted die, and curtain, and/or a metering means, e.g., bentblade, bevel blade, rod, roll, air knife, bar, gravure, size press(conventional or metering), and air brush. Preferably, the coating layeris applied by a bevel blade/short dwell coater. Preferably the totalcoat weight applied per side is about 1 to 4 g/m², more preferably about1.5 to 2 g/m². The solids level of the coating will typically range fromabout 35 to 55%, preferably 40 to 50%; a lower solids level is typicallyused to apply a coating at a low coat weight. Preferably the coating isapplied to both sides of the substrate to ensure that the printed imageson both sides of the printing sheet are of comparable quality. The imagereceptive coating may be applied to one or both sides of the substratein more than one coating layer. The coating layer is then dried, e.g.,by convection, conduction, radiation, or combinations thereof. Thetemperature of the paper surface during drying should not exceed theT_(g) of the hard polymer pigment or the hard polymer domain of thestructured polymer pigment if present in the image receptive coating.

[0071] The precoat layer is applied to the substrate prior to theapplication of the image receptive layer. The precoat layer is appliedin a similar manner as the image receptive layer as described above.Preferably, the precoat layer is applied by a bent blade/applicator rollcoater. The precoat may be applied in one or more layers. Preferably thetotal coat weight applied per side is about 5 to 15 g/m², morepreferably about 7 to 10 g/m².

[0072] The amount of precoat initially applied to the substrate dependssomewhat on the roughness of the underlying substrate. The precoat layermay not be necessary if the substrate has adequate smoothness such that,after application of the image receptive coating, the final printingsheet smoothness meets the desired levels. At a minimum, the precoatlayer generally should be thick enough to fill in the valleys of thesubstrate's surface after drying, taking into account that someshrinkage of the precoat will occur upon drying. On the other hand, iftoo much precoat is initially applied to the substrate, the substratemay absorb an excessive amount of water from the precoat, causing thefibers to swell and move within the substrate. Such fiber swelling andmovement can negatively affect smoothness. The solids level of theprecoat will typically range from about 55 to 70%. Preferably theprecoat is applied to both sides of the substrate. The precoat layer isthen dried, e.g., by convection, conduction, infrared, or combinationsthereof. The drying temperature for the precoat layer is not restricted.

[0073] As discussed above, if increased smoothness is desired, theprecoated substrate may be calendered before the image receptive layeris applied. Alternatively, an additional precoat layer may be applied.Subsequent precoat layers typically have negligible, or no, effect onthe substrate's tendency to absorb water and swell. Such absorptiontypically occurs during the first precoat application.

[0074] The printing sheet of the invention exhibits satisfactory levelsof gloss, smoothness, bulk, stiffness and opacity without the need forcalendering or other finishing steps. If calendering is desired it maybe performed after the image receptive coating has been applied anddried. The calendering apparatus may be a separate supercalender, anoff-line soft-nip calender, or an on-line soft-nip calendering unit. Thelevel of calendering performed on the sheet is dependent on the desiredproduct attributes, such as paper gloss and sheet bulk. Because theimage receptive coating layer is responsive to calendering, i.e.,develops gloss and smoothness easily, much lower operating conditions,e.g., temperature and pressure, are necessary during calendering toachieve the desired levels of gloss and smoothness. Preferably the nippressures range from about 40 to 90 kN/m, the temperature of the papersurface during calendering is preferably at least 5° C. lower than theT_(g) of the hard polymer pigment, and the incoming web moisture isabout 3 to 10%.

[0075] A brushing step may be performed after the image receptivecoating has been applied and dried. The brusher apparatus may be aseparate apparatus, or a process unit in a continuous line. Brushing maybe used to achieve the desired levels of paper gloss at a higher bulkthan may be achieved through calendering. Brushing intensity iscontrolled by three variables: brushing area, i.e., the surface area ofthe coated substrate in contact with the brushes; brushing force, i.e.,the tangential force applied by the brushes against the surface of thecoated substrate; and brush speed. Net specific brushing intensity iscalculated from the net power of the brusher motor, the substrate webspeed, and the width of the substrate web. If the brushing intensity istoo low, the desired level of gloss and smoothness may not be achieved.If the brushing intensity is too high, the surface of the imagereceiving layer may be damaged, e.g., scratches and streaks, and/or thesurface may coalesce to such an extent that complete film formationoccurs deleteriously affecting printing properties. Because the imagereceptive coating layer is responsive to brushing, i.e., develops glossand smoothness easily, much lower brushing intensity is necessary duringbrushing to achieve the desired levels of gloss and smoothness.

[0076] Suitable brushers typically have a plurality of brush rolls,e.g., four to eight brush rolls, and are commercially available from DOXMaschinenbau GmbH.

EXAMPLES

[0077] Table 1 below provides coating formulation information and finalproduct attribute data for two embodiments of the invention using PMMAas the hard polymer pigment and one comparison example with the coatingcontaining polystyrene particles as the pigment. Two different particlessizes for hard polymer pigment were evaluated in Examples B and C. TheT_(g) of the PMMA pigment was approximately 128° C. and the T_(g) of thepolystyrene particles was approximately 100° C.

[0078]FIG. 2 graphs shear modulus, G′, as a function of temperature forExamples A, B, and C. The shear modulus of the pigments was determinedusing a dynamic mechanical analysis (DMA) instrument, Model RDS 11manufactured by Rheometric Scientific of Piscataway, N.J. Suitable DMAinstruments are also available from TA Instruments of New Castle, Del.To measure shear modulus, a sample of the pigment material is preparedby drying the material on a hot plate at 50° C. and by subjecting theresulting dried material to compression molding, typically at atemperature of 220° C. and a pressure of 1,400 kg/cm² for one hour. Thedimensions of the resulting rectangular sample are approximately 2 mm×13mm×50 mm. The sample is held in the instrument between two clampstypically spaced 50 mm apart. The analysis is preformed using torsionrectangular geometry at a frequency of 1 rad/sec through a temperaturerange of 0 to 50° C., at 3° C. increments. Torsional force is applied tothe sample such that the strain is in the linear viscoelastic regime,typically a strain rate of about 0.01 to 0.05%. The strain, ordisplacement, of the sample is measured and the shear modulus iscalculated from the stress-strain curve and the sample dimensions. Theshear modulus values provided in Table 1 were calculated at atemperature of 21° C., to simulate the temperatures experienced bycoated papers during the printing and subsequent processes.

[0079] Prior to application of the image receptive coatings, theexamples in Table 1 were precoated and calendered on pilot equipmentusing a substrate manufactured on a commercial scale paper machine. Thesubstrate had a basis weight of 105 g/m². The precoat included 100 partsprecipitated calcium carbonate, 12 parts carboxylated styrene-butadienelatex as the film forming binder, and other coating additives. Theprecoated substrate was dried to a moisture content of approximately 4%using a combination of infrared and air flotation dryers. The precoatedsubstrate was then soft-nip calendered one nip per side, at atemperature of 120° C. and a nip pressure of 140 kN/m.

[0080] The image receptive coatings included 30 parts of astyrene-butadiene latex per 100 parts by weight of pigment as the filmforming binder. The image receptive coatings were applied to one side ofthe precoated substrate using a laboratory bevel blade coater. The coatweight of the image receptive coatings was 2.25±1.0 g/m². The coatinglayer was dried for 5 seconds under an infrared heating unit positionedapproximately 8 cm from the coated surface. The surface temperature ofthe heater was set at 315° C. to ensure the coated surface temperaturedid not exceed 50° C. The surface temperature of the sheet was measuredusing a temperature sensitive label applied to the sheet before coatingapplication. Temperature sensitive labels are commercially availablefrom Omega Engineering, Inc. of Stamford, Conn., e.g., Eight PointIrreversible Temperature Indicators, Catalog No. 8MA-100/38. The coatedsheets were not calendered. TABLE 1 Formulation: Example A Example BExample C PMMA pigment (parts) 0 100 100 Polystyrene pigment (parts) 1000 0 Pigment particle size (Å) 2,200 1,900 940 Shear Modulus (dynes/cm²)4.95 × 10⁹ 14.7 × 10⁹ 13.3 × 10⁹ Product Attributes: 75° Gloss 62.0 57.569.6 PPS Smoothness (10 kg Soft) 0.81 0.56 0.62 Porosity (Sheffield)(ml/min) 6.1 4.7 4.1 Burnish Resistance Very Poor Very Good Very Good

[0081] The measurements for 75° gloss were performed in accordance withTappi Method T-480 om-99. A glossier coated paper surface is indicatedby a higher 75° gloss value. Parker Print Surf (PPS) is a measure of thesmoothness of the surface, with a lower value indicating a smoothersurface. PPS measurements were performed according to Tappi Method T-555om-94. The Sheffield porosity measurements were performed in accordancewith Tappi Method T-547 om-97, using a 27 mm diameter orifice. HigherSheffield porosity values indicate greater flow through the paper, whichtypically provides improved ink setting and blister resistance.

[0082] The following test was developed to measure burnish resistance ofa coated sheet. In brief, a weighted sled is dragged across the surfaceof a sample and the resulting burnish mark, if any, is evaluated. Theweighted sled is a steel cylinder having a diameter of 6.6 cm, a heightof approximately 12 cm and a weight of 3.07 kg. One end of the cylinderis covered with napped polishing cloth (e.g., Buehler Microcloth,catalog number 40-7218). A thin wire is attached to the side of thecylinder with a screw eye. The test is performed on a paper samplehaving dimensions of at least 8 cm by 25 cm wherein the machinedirection of the paper is parallel with the short dimension. The sampleis attached to a flat, smooth, incompressible surface. The sled isplaced on one end of the paper sample with the polishing cloth againstthe surface of the test sample. The sled is pulled by the wire along thelong dimension of the test sample at a constant rate of 2 cm/sec.

[0083] The burnish resistance of a sample is determined by visuallycomparing the sample to a set of standards. The standards are rated verypoor, poor, fair, good, very good or excellent, with a rating ofexcellent signifying no burnishing and a rating of very poor signifyingsignificant, and unacceptable, burnishing. Five handsheet samples foreach example in Table 1 were prepared and rated against the set ofstandards. Samples with low burnish resistance tend to have burnishratings of poor and very poor. Samples exhibiting resistance toburnishing tend to have burnish ratings of fair to very good. Coatedprinting papers having coatings consisting primarily of mineral pigmentstypically have burnish ratings of good to excellent.

[0084] The product attributes provided in Table 1 are typically used todifferentiate between coated printing sheets for offset printing.Generally, glossy coated printing papers suitable for offset printingexhibit 75° gloss greater than about 65.0 and PPS smoothness less thanabout 1.20. The data in Table 1 indicates that Examples B and C withhard polymer pigment in the image receptive coating exhibit the productattributes generally desired for offset printing and very good burnishresistance. The 75° gloss level of Example B demonstrates the effect ofthe larger particle size of the hard polymer pigment. As discussedabove, hard polymer pigments having larger particle size may be used forlow gloss grades of paper. Example A with polystyrene particles as thepigment does not exhibit acceptable burnish resistance.

[0085] Table 2 below provides coating formulation information and finalproduct attribute data for three embodiments of the invention using thesame grade of PMMA used in Example C as the hard polymer pigment and amodified styrene/butadiene latex as the film forming binder. Example Falso contains a structured polymer pigment as an additional pigment. Thestructured polymer pigment is composed of primarily methyl methacrylateand butadiene.

[0086] The examples in Table 2 were produced on a pilot coater using asubstrate manufactured on a commercial scale paper machine. Thesubstrate had a basis weight of 84 g/m². A precoat was applied to eachside of the substrate at a coat weight of approximately 10 g/m² perside. The precoat included 100 parts precipitated calcium carbonate, 12parts carboxylated styrene-butadiene latex as the film forming binder,and other coating additives. The precoated substrate was dried to amoisture content of approximately 4% using a combination of infrared andair flotation dryers. The precoated substrate was then soft nipcalendered one nip per side, at a temperature of 120° C. and a nippressure of 88 kN/m. The smoothness of the precoated substrate for eachexample in Table 2 was 1.3 to 1.6.

[0087] The image receptive coatings were applied to both sides of theprecoated substrates using a short dwell bevel blade coater. The coatweight of the image receptive coatings was 2.5±11.0 g/m². The resultingcoated papers were dried to a moisture content of approximately 4% usinga combination of infrared and air flotation dryers. The coated paperswere not calendered after application of the image receptive coatings.TABLE 2 Formulation: Example D Example E Example F Hard Polymer Pigment(parts) 100 100 62 Structured Polymer (parts) 0 0 38 Film Forming Binder(parts) 25 56 18 Product Attributes: Specific Volume (cm³/g) 0.93 0.970.98 Porosity (Sheffield) (ml/min) 21.8 3.4 26.3 PPS Smoothness (10 kgSoft) 0.95 1.03 1.09 L&W Stiffness (MD/CD)¹ 36.6/18.2 26.8/18.436.1/18.3 75° Gloss 70.1 72.2 66.8 Brightness 88.6 88.5 88.8 Opacity94.3 94.5 94.8 75° Ink Gloss (Air Dry) 65.3 88.7 68.8 Burnish ResistanceFair Good Good

[0088] Several of the tests listed in Table 2 are described above withreference to Table 1. Specific volume is calculated by dividing thecaliper, i.e., thickness, of the sheet by the basis weight of the sheet.Basis weight is the weight of a specified area of paper, typicallyexpressed in grams per square meter. Specific volume provides anindication of the bulk or density of a printing paper. A dense sheetwill exhibit a low specific volume value while a bulkier sheet willexhibit a high specific volume value for the same basis weight. Specificvolumes of 0.75 to 0.90 are typically exhibited by coated printingpapers. Because printing sheets are generally priced by basis weight,the ability to provide sheets with higher specific volume is generallydesirable because it may decrease the cost of the paper to the customer.Because they have a high specific volume, the printing sheets of theinvention with a basis weight of, for example, 90 g/m², provide some ofthe physical attributes of a sheet with a higher basis weight, e.g., 100g/m², such as stiffness and bulk. In addition, the printing sheets ofthe invention maintain other desirable product attributes such as glossand smoothness. Therefore, the customer pays a lower price for the lowerbasis weight but receives the product quality attributes of a higherbasis weight sheet.

[0089] The measurements for Lorentzen & Wettre (L&W) stiffniess wereperformed according to method DIN-53-1221. L&W stiffness is the bendingforce which results when a sample is subjected to a bending angle of15°, and a higher value indicates greater stiffness. Opacity andbrightness measurements were performed according to Tappi Methods T-425and T-452 om-87, respectively.

[0090] The 75° gloss measurements for paper gloss and ink gloss wereperformed according to Tappi Method T-480 om-99. For gloss, the higherthe value, the greater is the gloss of the paper or printed image.Samples for the ink gloss measurements were prepared by printing thepaper sample with a solid image on a laboratory printing press, usingthe dry offset method. The resulting ink film was dried for 24 hours ina controlled environment typical of pressroom conditions, e.g., 21° C.temperature and 50% humidity. The dried ink film was then measured for75° gloss. Because laboratory printing equipment, test inks and samplepreparation methods may vary, the ink gloss values in Table 2 arepresented for comparison purposes only. Practitioners skilled in the artof printing are aware of how to prepare an ink film on paper formeasuring ink gloss.

[0091] The final product attribute values shown in Table 2 areconsidered very good for printing sheets without the use of calendering.The lower Sheffield porosity exhibited by Example E is caused by theincreased amount of film forming binder in the image receptive coating.Because Sheffield porosity values can also be affected by processingvariables other than the composition of the image receptive coating,comparisons of the Sheffield porosity of experimental samples may bevalid only within the context of a particular experiment. The values ofSheffield porosity provided in Table 2 are considered acceptable forexperimental samples. The embodiments of the invention provided in Table2 exhibit fair to good burnish resistance.

[0092] Table 3 below provides process information and product attributedata for four embodiments of the invention. Product attribute data isprovided for both the precoated intermediate product and the finalproduct. The image receptive coatings include 100 parts of the samegrade of PMMA used in Example C as the hard polymer pigment and 56 partsof a modified styrene-butadiene latex as the film forming binder. Theexamples in Table 3 were produced on a pilot coater using a substratemanufactured on a commercial scale paper machine. The substrate had abasis weight of 56 g/m². A first precoat layer was applied to each sideof the substrate at a coat weight of approximately 7.5 g/m² per side.

[0093] The precoat layer for Examples G and H included 75 partsstructured clay, 25 parts precipitated calcium carbonate, 15 partscarboxylated styrene-butadiene latex as the binder, and other coatingadditives. The precoat layer for Examples I and J included 100 partsground calcium carbonate, 15 parts carboxylated styrene/butadiene latexas the binder, and other coating additives.

[0094] Examples I and J were further coated with a second precoat layer.The second precoat layer was applied to each side of the substrate at acoat weight of approximately 4.5 g/m² per side. The second precoatincluded 73 parts structured clay, 23 parts precipitated calciumcarbonate, and 4 parts solid plastic pigment as the pigments, 10 partscarboxylated styrene-butadiene latex and 3 parts ethylated starch as thebinder, and other coating additives. The precoated substrates for allexamples were then dried to a moisture content of approximately 4% usingair flotation dryers.

[0095] The precoated substrates of Example H and Example J were soft-nipcalendered one nip per side, after the first precoat layer and secondprecoat layer respectively. Example H was calendered at a temperature of150° C. and a nip pressure of 140 kN/m, while Example J was calenderedat a temperature of 150° C. and a nip pressure of 88 kN/m.

[0096] The image receptive coatings were applied to both sides of theprecoated substrates using a short dwell bevel blade coater. The coatweight of the image receptive coatings was 2.2±1.2 g/m² per side. Theresulting coated papers were dried to a moisture content ofapproximately 4% using air flotation dryers. The coated papers were notcalendered after application of the image receptive coatings. TABLE 3Example G Example H Example I Example J Process Conditions: 1^(st)Precoat Weight (g/m², per side) 7.5 7.5 7.5 7.5 2^(nd) Precoat Weight(g/m², per side) N/A N/A 4.5 4.5 Precoat Nip Pressure (kN/m) N/A 140 N/A88 Precoat Attributes:¹ Specific Volume (cm³/g) 1.10 0.97 1.00 0.92Porosity (Sheffield) (ml/min) 40.7 24.9 13.1 10.2 PPS Smoothness (10 kgSoft) 3.39 1.38 2.27 1.56 Final Product Attributes: Basis Weight (g/m²)75.9 73.6 83.0 85.1 Specific Volume (cm³/g) 1.03 0.97 0.96 0.91 Porosity(Sheffield) (ml/min) 1.73 2.8 0.33 0.1 PPS Smoothness (10 kg Soft) 1.460.88 1.22 1.15 L&W Stiffness (MD/CD)² 11.6/6.3 8.9/4.9 13.6/7.5 12.1/7.675° Gloss 59.7 72.6 70.2 79.1 Brightness 85.7 85.3 86.4 86.8 Opacity90.2 89.2 91.1 90.5 75° Ink Gloss (Heat Set) 79.5 90.5 87.8 91.1 BurnishResistance Good Fair Good Good

[0097] Most of the tests listed in Table 3 are described above withreference to Tables 1 and 2. Samples for the heat set ink glossmeasurements were prepared as described above for the air-dry ink glossmeasurements. The resulting ink film was dried under conditionssimulating an oven used in web offset printers, e.g. the printed paperis heated to a temperature of 135° C. within 3 seconds and immediatelyremoved from the heat source. The ink film was further dried for 24hours in a controlled environment typical of pressroom conditions, e.g.,21° C. temperature and 50% humidity. The dried ink film was thenmeasured for 75° gloss. As in the case of the air-dry ink gloss valuesprovided in Table 2, the heat set ink gloss values in Table 3 arepresented for comparison purposes only.

[0098] The examples provided in Table 3 demonstrate different methodsthat may be used to manufacture the coated printing sheet of theinvention. The final product attribute values shown in Table 3 areconsidered very good for printing sheets without the need to calenderthe image receptive coated product. Examples I and J demonstrate thatcertain final product attributes may be improved with the use of asecond precoat layer prior to application of the image receptive coatinglayer. Examples H and J demonstrate that calendering of the precoatlayer immediately underlying the image receptive layer may also improvefinal product attributes. As discussed above, comparisons of theSheffield porosity of experimental samples may be valid only within thecontext of a particular experiment. The Sheffield porosity valuesprovided in Table 3 are considered acceptable. Embodiments of theinvention provided in Table 3 exhibit fair to good burnish resistance.

[0099] Other embodiments are within the claims. Various modifications ofthis invention will become apparent to those skilled in the art withoutdeparting from the scope or spirit of this invention.

What is claimed is: 1-27. (cancelled).
 28. A method of manufacturing aprinting sheet comprising: a) applying an image receptive coating,comprising a hard polymer pigment having a shear modulus of at least5.0×10⁹ dynes/cm² and a film forming binder, to at least a first surfaceof a substrate; and b) drying the image receptive coating layer.
 29. Amethod of claim 28 wherein the hard polymer pigment has a shear modulusof at least 10.0×10⁹ dynes/cm².
 30. A method of claim 28 wherein apressing step is performed on the substrate before the image receptivecoating application step at a moisture level ranging from 20 to 60% andat a temperature of at least 100° C.
 31. A method of claim 28 whereinthe substrate, prior to application of the image receptive coating, hasa smoothness of less than about 3.5 μm.
 32. A method of claim 31 whereinthe smoothness of the substrate is less than about 2.0 μm.
 33. A methodof claim 32 wherein the smoothness of the substrate is less than about1.5 μm.
 34. A method of claim 28 wherein a precoat layer applicationstep and a precoat layer drying step are performed before the imagereceptive coating application step.
 35. A method of claim 34 wherein theprecoat layer comprises a binder and a pigment selected from the groupconsisting of kaolin, calcined clay, structured clay, ground calciumcarbonate, precipitated calcium carbonate, titanium dioxide, aluminumtrihydrate, satin white, hollow sphere plastic pigment, solid plasticpigment, silica, zinc oxide, barium sulfate and mixtures thereof.
 36. Amethod of claim 35 wherein the pigment has a monodisperse particle sizedistribution.
 37. A method of claim 36 wherein the monodisperse pigmentis selected from the group consisting of precipitated calcium carbonate,hollow sphere plastic pigment and mixtures thereof.
 38. A method ofclaim 34 wherein the precoat layer has a total dried coat weight perside of about 5 to 15 g/m².
 39. A method of claim 28 or 34 wherein acalendering step is performed before the image receptive coatingapplication step.
 40. A method of claim 28 wherein a calendering step isperformed after the image receptive coating drying step.
 41. A method ofclaim 40 wherein the calendering step is performed at a nip pressure ofless than 90 kN/m.
 42. A method of claim 41 wherein the calendering stepis performed at a paper surface temperature of at least 5° C. lower thanthe glass transition temperature of the hard polymer pigment.
 43. Amethod of claim 28 wherein a brushing step is performed after the imagereceptive coating drying step.
 44. (canceled)
 45. A method of claim 28wherein the hard polymer pigment is essentially non-film forming andremains in a form of discrete roughly spherical solid particles in theimage receptive coating layer.
 46. A method of claim 28 wherein the hardpolymer pigment exhibits a glass transition temperature of at least 80°C.
 47. A method of claim 28 wherein the hard polymer pigment is selectedfrom the group consisting of poly(methyl methacrylate),poly(2-chloroethyl methacrylate), poly(isopropyl methacrylate),poly(phenyl methacrylate), polyacrylonitrile, polymethacrylonitrile,polycarbonates, polyetheretherketones, polyimides, acetals,polyphenylene sulfides, phenolic resins, melamine resins, urea resins,epoxy resins, and alloys, blends, mixtures and derivatives thereof. 48.A method of claim 47 wherein the hard polymer pigment has a homogenouscomposition comprising poly(methyl methacrylate) particles.
 49. A methodof claim 28 wherein the hard polymer pigment particles have a particlesize of less than about 2,000 angstroms (Å).
 50. A method of claim 28wherein the image receptive coating comprises at least 30 parts byweight of the hard polymer pigment, based on 100 parts by weight oftotal pigment.
 51. A method of claim 50 wherein the image receptivecoating comprises at least 80 parts by weight of the polymer pigment,based on 100 parts by weight of total pigment.
 52. A method of claim 28wherein the film forming binder is selected from the group consisting oflatex, starch, polyacrylate salt, polyvinyl alcohol, soy, casein,carboxymethyl cellulose, hydroxymethyl cellulose and mixtures thereof.53. A method of claim 28 wherein the image receptive coating furthercomprises a pigment selected from the group consisting of structuredpolymer pigment, kaolin, calcined clay, structured clay, ground calciumcarbonate, precipitated calcium carbonate, titanium dioxide, aluminumtrihydrate, satin white, hollow sphere plastic pigment, solid plasticpigment, silica, zinc oxide, barium sulfate and mixtures thereof.
 54. Amethod of claim 53 wherein the image receptive coating further comprisesstructured polymer pigment consisting of a soft domain having a glasstransition temperature of less than about 50° C. and a hard domainhaving a glass transition temperature of greater than about 55° C.
 55. Amethod of claim 28 wherein the image receptive coating has a total driedcoat weight per side of about 1 to 4 g/m².